I think you are baiting me with the suggestion of tariffs. I'm not sure there was a
major issue. States' Rights is a broad topic, but I don't believe Southerners wanted a bunch of Damnyankees telling them what to do--period. I think most Southerners honestly believed that any state could pull out of the Union any time it chose to do so--many Northerners believed this also. Economics was an important factor. Pride and jealousy on both sides was present and a factor--the Yankees were know-it-all snobs, and the Southerners were ignorant farmers be presumptuous and asserting themselves above their station. Each side looked down its nose at the other. Slavery was an issue, but when you consider the percentage of Southerners who fought in the war who owned slaves, it could not have been the major issue. Why would I, a person who does not own slaves, fight for your right, as a slaveholder, to own slaves?--just doesn't make sense. And then, there is always the fact that people just resist change--any change, whatever it is--especially rural, independent people.
The sad thing about it is that Lincoln was obsessed. If he had let the South go, it would have eventually rejoined the Union. Also, the southern states would have eventually eliminated slavery on their own. Then we would not have all the animosity we have today. I think that if the North and South had stayed apart for a while, there may not have been a Spanish-American War--or maybe even a WWI. The balance of power in Europe would have been different.